It is desirable to obtain a flowable mass of polymer plastic materials which can be utilized in extrusion and injection molding apparatus, and the like. The flowable plastic material is usually formed from particulate material, i.e., polymer granules which must be softened to form a flowable mass thereof. As is well known in plasticating extruders, for example, the plastic granules are supplied to a screw-type extrusion pump which softens compacted granules by pressure which creates frictional heat among the compacted particles which are supplied thereto so that at the molding end of the extrusion screw the particles are converted to a flowable mass of plastic which can be extruded through an appropriate die.
Such plasticating extruders are not always effective because the heat efficiency is relatively low and the process can adversely affect the mechanical properties of the plastic material. Other plasticizing extruders, such as that shown by U.S. Pat. No. 3,354,501 of Bachman et al., use external steam generators to heat the barrel of a ram-operated extrusion device and suffer from the same problem of obtaining a sufficiently uniform mass of flowable material.
If, in such extruder devices, the granules are, alternatively, pre-heated to a flowable state before supplying them to the extruder by applying heat to a compacted mass of such particles in an appropriate container, the low thermal conductivity of the plastic particles tends to prevent a uniform heating of the granules if relatively high temperature heating is used since, by the time the interior particles are heated, the exterior particles are at such a high temperature that the plastic tends to degrade. If relatively low temperature heating is used to avoid such a problem, the low temperature heating that is required leads to unacceptably long processing time which is impractical for use in commercially feasible systems. Accordingly, a uniform flowable mass of the material is difficult, if not impossible, to achieve in a practical manner by pre-heating a compacted mass thereof.
Another method for heating plastic granules is described in U.S. Pat. No. 3,163,888, issued to R. Shattuck, in which a chamber, containing purposely immovable granules, has perforated walls through which a warm gas under low pressure is introduced. Apart from the difficulty in achieving uniform heating of the granules which are in contact with each other within the chamber, the mass which is transferred from the chamber tends to contain gas which becomes entrapped within the interstices between the granules. Such entrapped gas alters the homogeneity of the flowable mass which is required in many applications and tends to weaken the material which ultimately forms a molded article, for example.